This invention relates to a loudspeaker system which includes dynamic speakers, and more particularly, to a loudspeaker system which includes at least two movable voice coils mounted on a single rigid coil form in operative association with a pair of amplifiers.
A loudspeaker system of this general type has been disclosed in U.S. Pat. No. 2,926,221 issued to W. A. Kagdis on Feb. 23, 1960 and entitled "Loudspeaker Construction". In this '221 patent, a pair of spaced apart magnetic circuits is each provided with a respective air gap. A rigid coil form extends between the air gaps, and the respective ends of the coil form extend into the respective gaps and have respective voice coils thereon.
Similar loudspeakers are disclosed in U.S. Pat. No. 3,686,446 to J. W. Manger entitled "Push-Pull Moving Coil Loudspeaker having Electromagnetic Centering Means" and in U.S. Pat. No. 3,055,991 to R. Guss entitled "Loudspeaker", the former also disclosing a number of prior art circuits used to drive the voice coils. In particular, Manger, supra, discloses a number of circuits suitable for driving the pair of voice coils in push-pull. In particular, a pair of transistors of like conductivity type or a pair of complementary transistors are provided to effect the driving of the pair of voice coils in push-pull. In each case, it appears that large D.C. currents must flow in the transistors and in the voice coils, without regard to signal levels, at all times in order to assure that clipping and distortion will not result.
Moreover, circuits which require relatively large D.C. current flow at all times through the voice coils and the transistors are inefficient. Furthermore, the voice coils and particularly the transistors must either have relatively high heat dissipating capacity or be provided with large heat sinks which are both bulky and expensive. In some instances, other circuitry which removes the D.C. component from the voice coils by feeding the voice coils via capacitor or transformer circuits, can be provided. This has a disadvantage that additional circuit components are required and does not solve the problem of heat generation in the transistors themselves and its associated inefficiency.
One possible known solution to the problem is to operate a transistor amplifier as class B amplifiers, in effect allowing only one of the transistors of a pair to conduct at any given time. One of the problems of operating transistors in an audio circuit as Class B amplifiers is that a considerable amount of distortion is likely to be produced. It is much better, if possible, to operate audio amplifiers as Class A amplifiers which produce less distortion. It is also known that when complementary transistors are utilized to effect a push-pull operation, that the turning on and off of these transistors produces cross-over distortion.